The present invention relates to a belt-drive continuously variable transmission including a primary pulley and a secondary pulley each having a pair of opposed sheave surfaces, and a belt wound on the pairs of opposed sheave surfaces to connect the primary pulley and the secondary pulley with each other.
The belt-drive continuously variable transmission is operative to ensure a necessary belt clamping force by applying a hydraulic pressure to the primary pulley during high speed running and maintain a highest transmission ratio against variation in transmission torque. In this condition, a primary pressure applied to the primary pulley becomes maximum within a transmission unit, and therefore, a line pressure to be regulated on the basis of a pump discharge pressure from an oil pump must be increased to at least the primary pressure. For this reason, during high speed cruising in which the high speed running is maintained for a long period of time, torque for driving the oil pump cannot be reduced. Accordingly, in an engine-equipped vehicle in which the torque for driving the oil pump is obtained from an engine, it is not possible to enhance fuel economy.
In order to solve the above problem, there have been proposed transmission ratio lock mechanisms for a belt-drive continuously variable transmission which is adapted to lock a primary moveable sheave relative to a primary fixed sheave in a position where a predetermined transmission ratio is attained. Japanese Patent Application Unexamined Publication No. 2008-51154 discloses a hydraulically operated transmission ratio lock mechanism that is adapted to lock a primary moveable sheave relative to a primary fixed sheave in a position where a predetermined transmission ratio is attained, by sealing a hydraulic pressure in a primary pressure chamber of the primary pulley. Japanese Patent Application Unexamined Publication No. 2006-170387 discloses a meshing transmission ratio lock mechanism that is adapted to lock a primary moveable sheave relative to a primary fixed sheave in a position where a predetermined transmission ratio is attained, by using meshing engagement of the primary moveable sheave.